Solar Collector

ABSTRACT

A solar collector is provided that concentrates solar energy for heating a medium such as air. The solar collector includes a housing with a first end wall, a second end wall and a cavity therebetween. A receiver tube is extended through the first end wall on a first end and extending longitudinally within the cavity. A receiver core member made from a metal foam material is disposed within the receiver tube. At least one mirror is arranged within the cavity opposite the receiver tube, the at least one mirror being arranged to reflect light onto the receiver tube. In one embodiment, an automated cleaning system is arranged to clean the collector cover. In another embodiment, the mirror is made from a dielectric mirror.

CROSS REFERENCE TO RELATED APPLICATIONS (IF APPLICABLE)

This application claims priority to U.S. provisional application Ser. No. 61/369,876 entitled “Solar Collector” filed on Aug. 2, 2010 the contents of which are incorporated herein in its entirety.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to a solar collector and in particular to a solar collector having a receiver with an increased surface area.

Renewable energy sources are desirable as they allow for a low or non-polluting generation of energy that may then be converted into useful work. Renewable energy may come from a variety of sources including the sun, wind, and tidal sources for example. The sun provides a particularly desirable source of energy since it is widely available, has minimal impact on the environment and may used to create electrical energy directly via photovoltaic, or thermal energy that may then be used as a source of heat or used in the operation of an engine such as a steam turbine, a Brayton, a Rankine or a Stirling engine for example.

Devices that convert solar energy into thermal energy are sometimes referred to as solar collectors. A variety of solar collectors have been proposed, including flat plate collectors, parabolic trough collectors, parabolic dish collectors, tower collectors, Fresnel lens, Fresnel reflector and pyramid collectors. Some of the designs, such as flat plate collectors are non-concentrating, meaning that the absorption area is the same as the collection area. Other designs, such as the parabolic trough for example, concentrate the solar energy into a smaller area. Fresnel reflector receivers are similar to parabolic trough in that the parabolic shape is segmented and arranged in a planar manner. Each mirror segment has a different angle to focus the light coplanar to the other mirror segments. The concentrating type of design allows for higher temperatures to be achieved, which allows the solar collector to be used in applications such as power generation and process heating.

The parabolic tough operates by using a parabolic mirror to reflect and concentrate sunlight onto a focal point. An insulated tube is placed at the focal point to receive the reflected light. When a heat transfer fluid is flowed through the insulated tube, the sunlight heats the fluid that transfers the heat away from the collector. The heated fluid may be used in a number of different applications. In one application, the heated fluid is transferred to a boiler to make steam. The steam then in turn may be used to operate a turbine engine to generate electricity. In general, the coolant is made from a medium such as oil that allows temperatures up to 380° C. Other systems provide for the direct generation of pressurized steam within the solar collector. One disadvantage with these types of systems is that they are limited in the temperature that may achieve, and may also require high-pressure conduits and connectors within the system.

Accordingly, while existing solar collectors are suitable for their intended purposes, the need for improvements remains. In particular there remains a need for a system that provides for higher temperatures at lower pressures.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the invention, a solar collector is provided. The solar collector includes a housing having a first end wall, a second end wall and a cavity therebetween. A receiver tube extends through the first end wall on a first end and extending longitudinally within the cavity. A receiver core member is disposed within the receiver tube, the receiver core member being made from a porous material. At least one mirror is arranged within the cavity opposite the receiver tube, the at least one mirror being arranged to reflect light onto the receiver tube.

According to another aspect of the invention, another solar collector is provided. The solar collector includes a parallelepiped housing having a first end wall with at least one first opening, a second end wall and a pair of sides therebetween, the first end wall, the second end wall and the pair of sides defining a cavity. At least one first mirror is arranged on one side of the cavity. A cover is arranged on a side of the cavity opposite the at least one first mirror. A receiver is coupled to the first end wall and extends longitudinally within the housing, the receiver having a second opening within the cavity adjacent the second end wall.

According to yet another aspect of the invention, a method of operating a solar collector is provided. The method includes the step of reflecting light off a mirror towards a focal point. A receiver tube is provided at the focal point. A core made from a porous metal is provided within the receiver tube. The core is heated with the reflected light. A heat transfer medium flows through the core.

According to yet another aspect of the invention, a cleaning system for a solar collector having a cover is provided. The cleaning system includes a cleaning head having a fluid dispenser and a wiper, the cleaning head being arranged to fluidly couple the fluid dispenser to the cover when the cleaning head is in a first position and to contact the wiper with the cover when the cleaning head is in a second position. A motor is operably coupled to translate the cleaning head relative to the cover from a third position to a fourth position.

According to yet another aspect of the invention, a solar collector is provided. The solar collector includes a housing having a first end wall, a second end wall and a cavity therebetween. A receiver tube extends through the first end wall on a first end and extending longitudinally within the cavity. At least one mirror arranged within the cavity opposite the receiver tube, the at least one mirror being arranged to reflect light onto the receiver tube, wherein the at least one mirror is a dielectric mirror.

According to yet another aspect of the invention, a solar collector array is provided. A frame is rotatable about a first axis. A plurality of solar collectors is each pivotally coupled to the frame, each solar collector having a mirror configured to reflect light towards a focal point, a receiver tube positioned substantially at the focal point and a porous metal receiver core. A first device is operably coupled to rotate the frame about the first axis. A second device is operably coupled to rotate at least one of the plurality of solar collectors.

These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWING

The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view illustration of a solar collector in accordance with an embodiment of the invention;

FIG. 2 is a reverse perspective view illustration of the solar collector of FIG. 1;

FIG. 3 is a partial top plan view illustration of the solar collector of FIG. 1;

FIG. 4 is a cross sectional view illustration of a receiver for use with the solar collector of FIG. 1;

FIG. 5 is a top plan schematic view illustration of air flow through the solar collector of FIG. 1;

FIG. 6 is a side plan view, partially in section of solar collector in accordance with another embodiment;

FIG. 7 is a side plan view of a solar collector having a cleaning system in accordance with another embodiment;

FIG. 8 is a side plan view of the solar collector of FIG. 7 in a first mode of operation;

FIG. 9 is a side plan view of the solar collector of FIG. 7 in a second mode of operation; and,

FIG. 10 is a perspective view illustration of an array including a plurality of solar collectors of FIG. 1.

The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.

DETAILED DESCRIPTION OF THE INVENTION

Solar collectors allow the harnessing of solar energy to that it may be converted into useful work. Embodiments of the present invention provide advantages in the heating of a heat transfer medium such as air to very high temperatures. In one embodiment, a metal foam core is used to increase the heating surface area of the receiver to allow air to be heated to temperatures up to 500° C. Embodiments of the present invention provide further advantages in preheating of a heat transfer medium via radiant heating. Still further embodiments of the invention provide advantages in providing an automatic cleaning of the solar collector cover.

A solar collector 20 in accordance with an embodiment is illustrated in FIG. 1 and FIG. 2. The solar collector 20 includes a housing 22 having a first end wall 24 and a second end wall 26. A pair of side walls 28 connect the end walls 24, 26 to form a generally parallelepiped shape that defines a cavity area 30. The housing 22 is supported by a frame 32 having a plurality of transverse support members 34 that extend between a pair of rails 36. As will be discussed in more detail below, the support members 34 include a Fresnel shaped top surface.

The housing 22 may further have a cover (not shown for clarity) that encloses the frame 32 and the bottom portion of the housing 22. In the exemplary embodiment, the housing 22 is made from a carbon fiber and resin form or an aluminum sheet material and includes an insulating coating on the interior walls 38. The insulating coating may be a ceramic insulating paint or a low density fiber insulator for example.

The first end wall 24 includes a first opening 40 and a second opening 42. As will be discussed in more detail below, the openings 40, 42 allow a heat transfer medium, such as ambient air for example to be drawn into the cavity 30. In the exemplary embodiment, the openings 40, 42 may include louvers or filters (not shown for clarify) that minimize the entry of contaminants, such as dust or insects for example, into the cavity 30. In the exemplary embodiment, the housing 22 is 1 meter wide, 4 meters long and 0.4 meter deep. The openings 40, 42 are approximately 0.3 meters long and 0.1 meters wide.

On one side of the cavity 30, a mirror 44 is arranged on the Fresnel surface of support members 34. As will be discussed in more detail below, the Fresnel configuration of the mirrors 44 allows the reflection of light to a focal point within the housing 22. In the exemplary embodiment, the mirror 44 is formed from a plurality of individual planar mirrors 46 (FIG. 3). In the exemplary embodiment, 70 planar mirrors 46 are arranged in two sets of 35 mirrors on the support members 34 with a central opening 48 disposed between the two-sets of mirrors. It should be appreciated that the claimed invention should not be limited by the number of mirrors and that any number of mirrors may be utilized that allow for the reflection to a focal point while maintaining a suitable structural integrity. The planar mirrors 46 are sized to allow the mirror 44 to approximate a parabolic shape. The planar mirrors 46 extend the length of the cavity 30 and are 0.013 meters wide. It should be appreciated that while embodiments herein refer to flat mirror segments, the claimed invention should not be so limited, as curved mirrors 44 can also be used to concentrate the light to a focal point.

It should be appreciated that in embodiments having multiple mirrors 46, each mirror may reflect in a manner that does not provide a single focal point as would occur with a single contiguous parabolic mirror. Rather, these plurality of focal points may be substantially co-located. While the descriptions herein may refer to a single focal point, the claimed invention should not be so limited and the reference to a singular focal point also refers to a mirror having a plurality of focal points that are substantially co-located.

In the exemplary embodiment, the mirrors 46 are a highly reflective aluminum strip with protective coatings such as manufactured by Alumet Manufacturing, Inc. or have a plastic or glass substrate, such as polycarbonate for example, with a wide spectrum dielectric mirror deposited thereon. In the exemplary embodiment, the mirrors 46 are a 0.508 millimeter thick aluminum reflector with a coating to protect the mirror and having 95% reflectivity, such as a model Miro-Sun mirror produced by Anomet Inc for example. The dielectric mirror may include a plurality of layers of different materials, such as silica and silicon for example. The reflectivity of the dielectric mirror may have reflectivity efficiency of greater than 99.99% over a wide spectrum of light wavelengths. The mirrors 46 may also be made from a polycarbonate strip with a reflective film, such as Mirror Film manufactured by ReflecTech, Inc. In other embodiments, the mirrors 46 may also be made from glass mirrors (e.g. tin-chloride-silver-copper coating), polished aluminum, or polished stainless steel for example.

In the exemplary embodiment, the mirrors 46 are coupled at one end, such as adjacent first end wall 24 for example, to the frame 32 or housing 22. The mirrors 46 may also be coupled to the support members 34 in a manner to allow the mirrors 46 to freely move in the longitudinal direction to accommodate for thermal expansion of the mirror 44. It should be appreciated that in embodiments where the mirror 44 is a single contiguous member, the coupling of the mirror 44 within the solar collector 22 may provide for movement to accommodate thermal expansion in both the longitudinal and transverse directions.

Opposite the mirror 44, a cover member 50 is arranged to enclose the cavity 30. The cover member 50 is configured to allow light to pass through into the cavity 30 while minimizing heat transfer from the cavity 30 to the ambient environment. Therefore, the cover member 50 may be clear, transparent, semitransparent, or translucent. In the exemplary embodiment, the cover member 50 is made from a high transmissive glass or transparent plastic material, such as polycarbonate for example. The cover member 50 may be glazed, unglazed or have antireflective coatings applied. The cover member is sized to substantially conform to the shape of the housing 22. In one embodiment, the cover member 50 is removably coupled to the housing 22 to allow removal for maintenance and cleaning of the solar collector 20.

Arranged between the mirror 44 and the cover member 50 is a receiver tube 52. In the exemplary embodiment, the receiver tube 52 may be positioned between the openings 40, 42 adjacent the cover member 50. The receiver tube 52 is positioned to be substantially co-axial with a focal point of the parabolic shape of mirror 44. In one embodiment, the receiver tube 52 has a diameter of 30 millimeters, a wall thickness of 1-2 millimeters and is made from a transparent or semitransparent borosilicate glass. The receiver tube 52 extends through the first end wall 24 and is fluidly coupled to a conduit system 54 (FIG. 5). One end of the receiver tube 52 is adjacent the second end wall 26 such that a gap 56 (FIG. 3) is disposed between the open end 58 of receiver tube 52 and the second end wall 26. In the exemplary embodiment, a bracket 60 is coupled between the second end wall 26 and the receiver tube 52 to support the end of the receiver tube 52 adjacent the open end 58. In another embodiment, the receiver tube 52 is coupled directly to the second end wall 26 and the receiver tube 52 includes a plurality of holes to allow air to pass from the cavity 30 into the receiver tube 52.

Disposed within the receiver tube 52 is a receiver core 62. In the exemplary embodiment, the receiver core 62 is made from an open cell, porous, metal material, such as a metal foam for example as illustrated in FIG. 4. The metal foam material is porous having a plurality of pores 65 defined by an interconnected network of struts 67. These pores 65 allow a heat transfer medium to flow through the receiver core 62 while the struts 67 absorb and transfer heat from the light striking the receiver. The receiver core 62 may be made from an isotropic foam (substantially identical cell structure in all directions), a woven structure, or a honeycomb structure for example. The receiver core 62 may also be made from a powdered sintered structure, a fiber sintered structure, or a hollow sphere structure for example. In one embodiment, the receiver core 62 is made from a steel foam material such as a Duocell Foam product manufactured by ERG Materials and Aerospace Corporation. It should be appreciated that while the term metal foam is used herein, the receiver core 62 may also be made from a porous metal or a metal sponge material or metal wool. The use of a metal foam material provides advantages in increasing the surface area for absorbing solar energy.

In one embodiment, the metal foam receiver core 62 has 10,000 times the surface area of the receiver tube 52. It should be appreciated that the use of the metal foam receiver core 62 provides advantages in increasing the transfer of thermal energy to the heat transfer medium. In one embodiment, the receiver core 62 has a porosity of 20-80%. It should be appreciated that the receiver core 62 is sized to provide an increased amount of surface area while allowing the desired heat transfer medium to flow from the open end 58 to the conduit system 54. In the exemplary embodiment, the metal foam receiver core 62 is coated with a black oxide coating. The black oxide coating provides advantages in improving the black body radiation properties of the receiver core 62.

It should be appreciated that while embodiments describe the operation of the receiver tube 52 and receiver core 62 with the use of a parabolic mirror concentrator, the claimed invention should not be so limited. The receiver tube 52 and receiver core 62 may also be used in a solar collector having a Fresnel lens that focuses incoming light on the receiver tube directly rather than through reflection.

In operation, an array 63 of one or more solar collectors 20 is arranged in parallel to receive sunlight. The solar collectors 20 are fluidly connected in parallel to the conduit system 54 as shown in FIG. 5. At one end of the conduit system 54, a fan 64 is coupled to draw air through the conduit system 54 to a conduit system 66 that connects the conduit system 54 with a downstream process 68 or a thermal storage tank 70. The downstream process 68 may be, but is not limited to, a boiler, an expander, a space heating system or an absorption chiller for example. Coupled between each solar collector 20 and the conduit system 54 is a temperature sensor 72 and a damping valve 74. The temperature sensor 72 and damping valve 74 cooperate to 30 independently control the flow of air through each cavity 30 to maintain a desired operating temperature, such as greater than 500° C. for example.

In one embodiment, the temperature sensor 72 and damping valve 74 are coupled to a controller that adjusts the operating temperature based on an operating parameter such as the time of day, the season of the year, the weather, or the amount of ambient sunlight available for example. In another embodiment, the fan 64 is coupled to multiple arrays 63 and is a variable speed blower that controls the overall flow rate of the multiple arrays.

In one embodiment, the solar collectors 20 are coupled to a tracking system that allows two-axis tracking of the solar collectors 20 relative to the sun that allows the orientation of the solar collectors 20 to remain relatively fixed. The use of a two-axis tracking arrangement increases the amount of solar energy that may be absorbed by the solar collector 20. In another embodiment, the solar collectors 20 are arranged in a single axis tracking configuration wherein the solar collectors 20 are aligned on a north-south horizontal axis and are rotated during the course of the day. In one embodiment, the solar collectors 20 are grouped on a tracking system as an array 63 of five solar collectors having an area of 20 m². In this embodiment, the array may produce between 15-20 kilowatts of thermal energy.

The light enters the solar collector 20 through the cover 50 and reflects off of the mirror 44 towards a focal point that is substantially co-axial with the receiver tube 52. The reflected light converts to thermal energy when the light strikes the receiver tube 52 and receiver core 62 increasing the temperature of the receiver core 62. When the fan 64 is activated, air is drawn through the openings 40, 42 into the cavity 30 as indicated by arrows 76. Within the cavity 30, as the heat transfer medium, such as air for example, flows from the openings 40, 42 to the area 78 of the cavity 30 adjacent open end 58, the air is heated by radiant heat transfer from the receiver core 62. In the exemplary embodiment, the air temperature is increased from ambient temperature to approximately 100° C.

The pre-heated air is then drawn into the receiver tube 52 and receiver core 62 by the forced convection system created by fan 64, conduit system 54 and damping valve 74. It should be appreciated that the rate of flow of the air through the receiver core 62 will depend on the state of valve 74 and the speed of the fan 64. When the temperature of the air as measured by the temperature sensor 72 is low, the rate of flow will be low due to the valve 74 being substantially closed. As the temperature of the receiver core 62 increases, the state of the valve 74 is opened allowing the rate of air to increase until the desired operating temperature is reached. In the exemplary embodiment, the operating temperature of the air exiting the solar collector is greater than or equal to 500° C.

The heated air from each solar collector 20 is combined in the conduit system 54 and delivered to a downstream process 68, 70. These downstream processes may include storage tanks 70 having a means for storing the thermal energy, such as with molten salt for example, so the heat may be used at a later point in time. Other downstream processes 68 may include, but are not limited to: absorption chillers, direct space heating, domestic hot water heating, chemical process heat, food process heat, boilers for space heating, and boilers coupled to a steam turbine for generating electricity or a combination of the foregoing for example. In one embodiment, the conduit system 66 may be fluidly coupled to the openings 40, 42 to allowing reheating of the air to increase the efficiency of the solar collector 20. The reheating of the air may be advantageous during early morning, later afternoon, or winter season time periods, or at higher latitudes where the amount of solar energy received by the solar collector 20 may be lower.

It should be appreciated that while embodiments described herein refer to the heat transfer medium as being air, the claimed invention should not be so limited. In one embodiment, the system is a closed cycle system with the heat transfer medium being transferred back to the solar collectors 20 after transferring the thermal energy to the downstream processes 68, 70. In another embodiment, the heat transfer medium is an fluid oil. In this embodiment, the receiver tube 52 extends through the second end wall 26 rather than flowing through openings 40, 42. One skilled in the art will appreciate that the conduit systems, 54, 66 may include additional components (not shown) suitable for the designed heat transfer medium, such as pumps, shutoff valves, check valves mass flow sensors, pressure sensors and the like.

Another embodiment of the solar collector 20 is illustrated in FIG. 6. In this embodiment, the solar collector 20 includes a housing 22 having a cavity 30. The mirror 44 is arranged on one side of the cavity 30 and supported by the support members 34. The receiver tube 52 and receiver core 62 are arranged between the mirror 44 and the cover 50 substantially co-axial with the focal point of mirror 44. In this embodiment, the solar concentrator further includes a secondary reflection mirror 80. The secondary reflection mirror 80 may be coupled to the housing 22, such as at first end wall 24 and second end wall 26, or may be mounted to the cover 50 by a frame 82 as illustrated in FIG. 6. The secondary reflection mirror 80 reflects light from mirror 44 onto the receiver tube 52. The use of a secondary reflection mirror provides advantages in increasing the concentration of the solar concentrator 20. The use of the secondary reflector increases the concentration from 70 suns up to several hundred suns.

It is desirable for the cover 50 to remain clean and substantially free from contaminants, such as dirt and dust for example. The presence of contaminants may result in less efficiency for the solar collector 20 as the contaminants tend to reflect a portion of the sunlight rather than allow it to pass into the cavity 30. Typically, cleaning of solar concentrators was performed by a manual process which presented difficulties when the concentrators were located in a remote location, such as a rooftop for example, or could result in higher operating costs at large installations of solar concentrators.

Referring now to FIGS. 7-9, an automated cleaning system 90 for a solar concentrator 20 is illustrated. The cleaning system 90 includes a cleaning head 92 that is coupled to move along a rail 94 mounted adjacent the cover 50. In the exemplary embodiment, the cleaning system 90 includes a second rail (not shown) that supports the opposite side of the cleaning head 92. In the exemplary embodiment, the cleaning head 92 includes a motor 96 that engages the rail 94 to linearly translate or move the cleaning head 92 when activated by a controller 98 and a power supply 100. It should be appreciated that other types drive devices, such as a belt or pulley arrangement for example, may also move the cleaning head 92.

In the exemplary embodiment, the cleaning head 92 includes a fluid dispenser or sponge 102 and a wiper 104 that are mounted to a frame 106. A manifold 108 and cleaning solution reservoir 110 are fluidly coupled to the sponge 102. The sponge 102 is made from a suitable material that absorbs the cleaning solution and releases the cleaning solution under pressure. The wiper 104 is made from a material, such as rubber for example, that removes the cleaning solution from the cover 50 when the wiper 104 is placed in contact the cover 50 and slid over the surface. Both the sponge 102 and the wiper 104 extend substantially the width of the cover 50.

A controller 98 controls the cleaning system 90. Controller 90 is a suitable electronic device capable of accepting data and instructions, executing the instructions to process the data, and presenting the results. Controller 90 may accept instructions through user interface, or through other means such as but not limited to electronic data card, voice activation means, manually-operable selection and control means, radiated wavelength and electronic or electrical transfer. Therefore, controller 90 can be a microprocessor, microcomputer, a minicomputer, an optical computer, a board computer, a complex instruction set computer, an ASIC (application specific integrated circuit), a reduced instruction set computer, an analog computer, a digital computer, a molecular computer, a quantum computer, a cellular computer, a superconducting computer, a supercomputer, a solid-state computer, a single-board computer, a buffered computer, a computer network or a hybrid of any of the foregoing. Controller 90 may include a processor coupled to a random access memory (RAM) device, a non-volatile memory (NVM) device, a read-only memory (ROM) device, one or more input/output (I/O) controllers, and a local area network or communications interface device.

Controller 38 includes operation control methods embodied in application code. such as those that control the operation of the damping valve for example. These methods are embodied in computer instructions written to be executed by a processor, typically in the form of software. The software can be encoded in any language, including, but not limited to, assembly language, VHDL (Verilog Hardware Description Language), VHSIC HDL (Very High Speed IC Hardware Description Language), Fortran (formula translation), C, C++, Visual C++, Java, ALGOL (algorithmic language), BASIC (beginners all-purpose symbolic instruction code), visual BASIC, ActiveX, HTML (HyperText Markup Language), and any combination or derivative of at least one of the foregoing. Additionally, an operator can use an existing software application such as a spreadsheet or database and correlate various cells with the variables enumerated in the algorithms. Furthermore, the software can be independent of other software or dependent upon other software, such as in the form of integrated software.

In operation, the controller 98 determines that a cleaning cycle is desired. The controller 98 may make this decision based on a variety of factors, such as the time of day for example. In other embodiments, the controller 98 may include a sensor (not shown) that indicates the level of contaminants on the cover 50 and the controller 98 decides to initiate a cleaning cycle when the sensor signal exceeds a threshold for example. Once the controller 98 initiates a cleaning cycle, cleaning solution is delivered from the reservoir 110 to the manifold 108 where it flows into the sponge 102. The controller 98 then rotates the cleaning head 92 causing the sponge to move to a position where it contacts the top surface of the cover 50. The pivoting of the cleaning head 92 may be performed by the motor 96, or by a second actuator (not shown), such as a second motor or a solenoid for example. With the cleaning head 92 rotated to this first position, or simultaneously with this rotation, the motor 96 engages the rail 96 and translates the cleaning head 92 in the direction indicated by arrow 112 (FIG. 8). The movement of the cleaning head 92 continues until the cleaning head 92 reaches the opposite end of the cover 50. It should be appreciated that since the sponge 102 is in contact with the cover 50 and at least partially compressed, the cleaning fluid from the sponge 102 will be released onto the cover 50. The controller 98 may release additional cleaning fluid from the reservoir 108 as needed to maintain the desired amount of cleaning solution on the cover 50.

Once the cleaning head 92 has reached the opposite end of the cover 50, the controller 98 causes the cleaning head 92 to pivot in the opposite direction to a second position as shown in FIG. 9. In this second position, the wiper 92 is in contact with the surface of cover 50 with a desired amount of pressure to remove the cleaning fluid from the cover 50 when the cleaning head 92 is moved across the surface. With the cleaning head in this second position, the direction of the motor 96 is reversed causing the cleaning head to move in the direction indicated by arrow 114. As the cleaning head 92 transverses the cover 50, the cleaning fluid is removed from the cover 52 and the surface of the cover 50 is cleaned. Once the cleaning head 92 reaches the opposite end of the cover, the motor 96 disengages. In the exemplary embodiment, the rail 94 extends beyond the first end wall 24 allowing the cleaning head 92 to be positioned off of the cover 50 when not in use so as to not interfere with operation of the solar collector 20.

In another embodiment, the solar collector 20 is pivotally arranged in an array 63 mounted on a frame 120. A pair of poles 116, 118 supports the frame on a hinge-pivot 122. The hinge pivot 122 allows the array 63 to both rotate 124 about a longitudinal axis and pivot 126 about a transverse axis. In one embodiment, the pole 118 includes a screw jack that adjusts the elevation and rotates the frame 120 about the transverse axis. In one embodiment, a motor 128 on pole 116 rotates the frame 120 about the longitudinal axis. In the exemplary embodiment, another motor 130 is coupled to each of the solar collector 120 by a shaft 132. In the exemplary embodiment, a gearing arrangement 134 couples the shaft 132 to rotate each solar collector 120 about its respective pivot 136.

Each motor 128, 130 is coupled to receive a signal from one or more an optical light receivers 138 that finds the elevation and azimuth of the sun. The light receiver 138 has encoders that read the proper angle for both axis and relays a signal to the motors. Encoders on the frame and cavity confirm the proper angle coming from the signal. In response to the signals received from receivers 138, controllers associated with the motors 128, 130 activate the motors 128, 130 to rotate the frame 120 and solar collectors 20 to the desired alignment position to receive light from the sun. In one embodiment, a single receiver 138 provides a signal both motors 128, 130. In another embodiment, each solar collector 20 has an individual receiver 138 and motor that allows individual adjustment of each solar collector 20.

It should be appreciated that while reference to motors 128, 130 are used herein, this is for exemplary purposes and the claimed invention should not be so limited. The motors 128, 130 may be a brushless DC motor, a series-universal motor, a solenoid, a hydraulic motor, or a pneumatic motor for example. In other embodiments, the motors 128, 130 may be a manually operated device. In yet another embodiment, the motors 128, 130 may be one or more internal combustion engine type devices that are capable of operating multiple arrays 63.

In yet another embodiment, the pole 118 is shared with a second array 63. In one embodiment, the sharing of pole 118 may be repeated up to 10 times to provide for the mounting and positioning of fifty solar collectors 20. These ten arrays 63 may be coupled to a single motor 30 to allow a single motor to adjust the pivot of the solar collectors 20.

While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims. 

1. A solar collector comprising: a housing having a first end wall, a second end wall and a cavity therebetween; a receiver tube extending through said first end wall on a first end and extending longitudinally within said cavity; a receiver core member disposed within said receiver tube, said receiver core member being made from a porous material; and, at least one mirror arranged within said cavity opposite said receiver tube, said at least one mirror being arranged to reflect light onto said receiver tube.
 2. The solar collector of claim 1 wherein said first end wall includes at least one opening.
 3. The solar collector of claim 2 wherein said receiver tube has a second end opposite said first end, said second end being arranged a predetermined distance from said second end wall to define a gap.
 4. The solar collector of claim 3 wherein said receiver tube is made from borosilicate glass.
 5. The solar collector of claim 4 wherein said receiver core member is made from a steel foam material.
 6. The solar collector of claim 5 further comprising a cover coupled to one side of said housing adjacent said receiver tube.
 7. A solar collector comprising: a parallelepiped housing having a first end wall with at least one first opening, a second end wall and a pair of sides therebetween, said first end wall, said second end wall and said pair of sides defining a cavity; at least one first mirror arranged on one side of said cavity; a cover arranged on a side of said cavity opposite said at least one first mirror; and, a receiver coupled to said first end wall and extending longitudinally within said housing, said receiver having a second opening within said cavity adjacent said second end wall.
 8. The solar collector of claim 7 further comprising a porous metal core member disposed within said receiver between said second opening and said first end wall.
 9. The solar collector of claim 8 wherein: said porous metal core member is a metal foam material having a black oxide coating; and, said receiver is a tube made from borosilicate glass.
 10. The solar collector of claim 9 wherein said metal foam core member is made from a steel foam material.
 11. The solar collector of claim 7 further comprising a second mirror extending longitudinally within said cavity opposite said at least one first mirror, wherein said receiver is arranged between said at least one first mirror and said second mirror.
 12. A method of operating a solar collector comprising: reflecting light off a mirror towards a focal point; providing a receiver tube at said focal point; providing a core made from a porous metal within said receiver tube; heating said core with said reflected light; and, flowing a heat transfer medium through said core.
 13. The method of claim 12 further comprising providing a housing, said mirror being arranged on one side of said housing and said focal point being arranged on a second side of said housing, said housing having a first end wall and a second end wall, said receiver tube extending through said first end wall, wherein said receiver tube includes a first opening adjacent said second end wall.
 14. The method of claim 13 wherein said heat transfer medium is air.
 15. The method of claim 14 further comprising flowing air through at least one second opening in said first end wall.
 16. The method of claim 15 further comprising heating said air with radiated heat from said receiver tube.
 17. A solar collector comprising: a housing having a first end wall, a second end wall and a cavity therebetween; a receiver tube extending through said first end wall on a first end and extending longitudinally within said cavity; and, at least one mirror arranged within said cavity opposite said receiver tube, said at least one mirror being arranged to reflect light onto said receiver tube, wherein said at least one mirror is a dielectric mirror.
 18. The solar collector of claim 17 wherein said at least one mirror includes at least one layer of silica and at least one layer silicon.
 19. The solar collector of claim 18 wherein said at least one mirror has a reflectivity efficiency of greater than 99.99%.
 20. The solar collector of claim 19 further comprising a receiver core member disposed within said receiver tube, said receiver core member being made from a metal foam. 